CytoTrace Red CMTPX: Cell Tracing & Viability Staining Guide

CytoTrace Red CMTPX is a cell-permeant, red fluorescent dye used for long-term tracking of live cells in culture and in vivo, retained in cytoplasm through covalent binding to intracellular components after passive diffusion across the plasma membrane.

Product SKU

C1539664

Fluorescence Color

Red

Cell Permeability

Cell-permeant (passive diffusion)

Retention Mechanism

Covalent binding to intracellular amines and thiol groups

Compatible Applications

Live-cell fluorescence microscopy, flow cytometry, in vivo cell tracking

Reference Compound

HY-D1562 (MCE cross-reference)

Principle and Mechanism of Action

CytoTrace Red CMTPX belongs to the CellTracker family of chloromethyl dyes. The dye freely diffuses across intact plasma membranes of live cells owing to its lipophilic, cell-permeant structure. Once inside the cytoplasm, the reactive chloromethyl group forms stable thioether bonds with intracellular glutathione and other thiol- or amine-containing proteins. This covalent conjugation anchors the fluorescent adduct within the cell, preventing leakage to neighboring cells and ensuring signal retention through multiple cell divisions and fixation steps.

The red emission spectrum of CMTPX is well separated from green fluorophores such as FITC and GFP, making it highly suitable for multi-color experimental designs. Because labeling depends on adequate intracellular thiol concentrations, only metabolically active (live) cells are labeled efficiently, providing an intrinsic viability indicator at the time of staining.

Reagent Preparation

Stock solution: Reconstitute CytoTrace Red CMTPX in high-quality, anhydrous dimethyl sulfoxide (DMSO) to prepare a concentrated stock solution (typically 10 mM). Vortex briefly to ensure complete dissolution. Aliquot into single-use volumes and store at −20 °C protected from light. Avoid repeated freeze-thaw cycles, which may reduce dye reactivity.

Working solution: Dilute the stock in pre-warmed (37 °C), serum-free, phenol red-free culture medium to the desired working concentration (commonly 1–25 µM). Prepare the working solution immediately before use and protect from light. Do not dilute directly in serum-containing medium, as serum components may react with the chloromethyl group and reduce labeling efficiency.

Note: Exact working concentrations should be empirically optimized for each cell type, as optimal concentrations may vary. Concentrations that are too high can cause cytotoxicity.

Step-by-Step Staining Protocol

1. Cell preparation: Grow cells to 70–90% confluency in appropriate culture vessels. Remove culture medium and wash cells once with pre-warmed phosphate-buffered saline (PBS) or serum-free medium.
2. Staining: Add the freshly prepared working solution of CytoTrace Red CMTPX to cells. Incubate at 37 °C, 5% CO₂ for 15–45 minutes. Protect from light throughout incubation.
3. Wash: Aspirate the dye-containing medium. Wash cells gently 2–3 times with pre-warmed, complete culture medium (containing serum) to quench unreacted dye. Allow cells to incubate in complete medium for an additional 30 minutes at 37 °C to ensure complete intracellular conjugation.
4. Analysis or transfer: Cells are now ready for downstream applications. For flow cytometry, detach cells using a non-enzymatic dissociation reagent where possible to preserve surface markers. For microscopy, image cells directly or proceed to fixation with 4% paraformaldehyde if required.
5. Imaging parameters: Use a Texas Red, mCherry, or Cy3 filter set (excitation ~577 nm / emission ~602 nm, verify with instrument specifications) for optimal signal detection.

Suspension cells: Pellet cells by gentle centrifugation, resuspend in working solution, incubate as above, then wash by centrifugation.

Troubleshooting

Problem	Possible Cause	Suggested Solution
Weak or no fluorescence signal	Dye degraded; working concentration too low; insufficient incubation time	Prepare fresh working solution; increase concentration empirically; extend incubation to 45 min
High background / non-specific staining	Insufficient post-staining washes; dye concentration too high	Perform additional washes with complete medium; reduce working concentration
Cytotoxicity observed	Dye concentration too high; prolonged incubation	Reduce concentration; shorten incubation time; verify cell health before staining
Signal lost after passaging	Expected dilution with cell division (signal halves each division)	Use higher initial loading concentration; limit tracking to fewer generations
Dye transfer to neighboring cells	Incomplete conjugation; excessive dye concentration	Ensure adequate post-staining incubation in complete medium; reduce concentration

Storage, Safety, and Handling Notes

Storage: Store the lyophilized or solid product at −20 °C in a desiccated, light-protected environment. DMSO stock solutions should be stored at −20 °C and are stable for up to 6 months when properly aliquoted and protected from moisture and light. Avoid repeated freeze-thaw cycles.

Safety: CytoTrace Red CMTPX is for research use only (RUO) and is not intended for diagnostic or therapeutic use in humans or animals. Wear appropriate personal protective equipment (PPE) including laboratory coat, nitrile gloves, and safety glasses when handling. DMSO is readily absorbed through the skin and can carry dissolved compounds; exercise caution. Refer to the Safety Data Sheet (SDS) for complete hazard information before use.

Disposal: Dispose of all waste in accordance with institutional, local, and national regulations for chemical and biological waste.

Frequently asked questions

Can CytoTrace Red CMTPX be used to label cells for in vivo tracking after adoptive transfer?

Yes, CMTPX-type dyes are commonly used for short- to medium-term in vivo cell tracking following adoptive transfer. The covalent intracellular retention prevents dye transfer to host cells. However, signal will dilute with each cell division, so the tracking window is limited by proliferation rate.

Is CytoTrace Red CMTPX compatible with fixation and permeabilization for immunostaining?

CMTPX fluorescence is generally retained after standard aldehyde-based fixation (e.g., 4% paraformaldehyde) because the dye is covalently bound to intracellular proteins. Compatibility with permeabilization reagents (e.g., Triton X-100, saponin) should be validated empirically, as harsh detergents may partially extract the labeled proteins.

What fluorescent channels are appropriate for detecting CytoTrace Red CMTPX?

CytoTrace Red CMTPX is typically detected using filter sets designed for Texas Red, mCherry, or Cy3 fluorophores. On flow cytometers, it is commonly detected in the PE-Texas Red or PerCP-Cy5.5 channel depending on the instrument configuration. Always verify excitation and emission compatibility with your specific instrument.

Why should the working solution be prepared in serum-free medium?

Serum contains abundant free amines and thiol-containing proteins that can react with the chloromethyl group of CMTPX before it enters the cells. Preparing the working solution in serum-free medium ensures that the reactive group remains available for intracellular conjugation, maximizing labeling efficiency.

How does CytoTrace Red CMTPX differ from CellTrace Violet or CFSE?

CytoTrace Red CMTPX emits in the red spectrum, whereas CFSE emits green and CellTrace Violet emits in the violet/blue range, allowing multiplexed tracking of different cell populations simultaneously. All three are retention dyes that dilute with cell division, but their distinct spectral profiles make them complementary tools in co-culture and mixed-population experiments.

References

1. Molecular Probes / Thermo Fisher Scientific. CellTracker Probes for Long-Term Tracing of Living Cells. Product documentation (general CMTPX class information).
2. Haugland RP. The Handbook: A Guide to Fluorescent Probes and Labeling Technologies. 10th ed. Molecular Probes, Inc. (General reference for chloromethyl fluorescent cell trackers.)

Products are supplied for research and development use only. Not for use in humans, animals, diagnosis, or therapy.